DESCRIPTION (adapted from the application) Shigatoxin-associated hemolytic uremic syndrome (HUS) is a thrombotic microangiopathic disorder in which microvascular thrombosis is a leading cause of acute and chronic organ injury. Despite decades of investigation, the basic hemostatic pathways most active in causing microvascular thrombosis in HUS have not been identified. Many investigations have implicated either von Willebrand factor (vWF)-platelet interactions, or thrombin-mediated coagulation pathways as primary pathological mechanisms. Until now however, the relative contributions of these hemostatic pathways to microvascular thrombosis in HUS has been impossible to determine because of a lack of adequate experimental models. This project takes advantage of a new animal model of shigatoxininduced HUS, and two drugs selected for their specific and proven ability to inhibit vWF and thrombin-mediated hemostasis, to test the hypothesis that: 1) The activities of thrombin and/or vWF are critical components in the mechanism of microvascular thrombosis in shigatoxin-associated HUS, and 2) Inhibition of the activity of thrombin and/or vWF will reduce or prevent pathological manifestations of microvascular thrombosis in a canine model of shigtoxin-associated HUS. This project will use an experimental model derived from a naturally occurring disease in Greyhound dogs that closely recapitulates the pathological, clinical, and epidemiological features of human shigatoxin-associated HUS. The Greyhound model is reproducible, amenable to manipulation, and more economical than primate models. HUS will be induced in the dogs by injection with shigatoxin while they are simultaneously treated with aurintricarboxylic acid, a specific inhibitor of shear-dependent platelet-vWF thrombosis, or Refludan, a new FDA-licensed specific thrombin inhibitor, or both drugs. Laboratory parameters used in the management of human HUS, as well as histopathological studies, will be used to determine the effectiveness of the drugs in modulating microvascular thrombosis. In addition, an array of specialized studies will be used to determine how the drugs affect in vivo vWF mediated and thrombin-mediated hemostatic functions in the dogs. By determining how inhibition of vWF and/or thrombin activity effects the development of microvascular thrombosis in this model, critical groundwork will be laid for future investigations of the pathophysiology of microvascular thrombosis in HUS focused on the most relevant hemostatic pathways. In addition, prevention or reduction of adverse outcomes of microvascular thrombosis in this animal model may lead directly to possibilities for improved patient care in humans.